Degradation of cellular biopolymers such as proteins and polysaccharides takes place chiefly within the lysosome. The end- products of this degradation, viz., amino acids and monosaccharides, are presumed to exit the lysosome to the cytoplasm, where further metabolism or expulsion to the external medium occurs. To study the process of lysosomal transport, we have developed methods to load lysosomes of various cells with amino acids (e.g., cystine, tyrosine) or with a specific monosaccharide (viz., sialic acid) and to measure their rates egress from the organelle. Our studies of cystine egress from lysosomes of human polymorphonuclear leukocytes and of tyrosine from cultured rat thyroid cell lysosomes have revealed these processes to be carrier-ediated and stereospecific. The further demonstration that no egress of cystine could be detected from similarly loaded lysosomes from patients with the inherited disorder cystinosis indicated that this storage disease is due to a congenital defect of a specific lysosomal carrier. Similar studies on the egress of sialic acid from fibroblast lysosomes have suggested strongly that impaired lysosomal transport underlies another lysosomal storage disorder, free sialic acid storage disease. In addition to a carrier system specific for the lysosomal transport of tyrosine, preliminary evidence has indicated that lysosomes from cultured rat thyroid cells also possess a carrier for mono-iodotyrosine an end-product of the lysosomal catabolism of thyroglobulin.